object—an object.
generalized-boolean—a generalized boolean.
Returns true if object is of type simple-string
;
otherwise, returns false.
(simple-string-p "aaaaaa") → true (simple-string-p (make-array 6 :element-type 'character :fill-pointer t)) → false
(simple-string-p object) ≡ (typep object 'simple-string)